Method of forming an e-i magnetic core



Oct. 12, 1965 J. POPA 3,210,830

METHOD OF FORMING AN E-I MAGNETIC CORE Orlglnal Flied. Jan. 28, 1959 5 sheetsheet 2 oct. 12, 1965 I J. POPA 3,210,830

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United States Patent O and this application `lune 26, 1962, Ser. No. 205,347 3 Claims. (Cl. 29--155.61)

The present application is a division of my application Serial No. 789,548, filed January 28, 1959, now Patent No. 3,077,570, issued February 12, 1963, and assigned to the assignee of the present application.

This invention relates to inductive devices, and to a method for forming magnetic cores for inductive devices such as are used for ballasting mercury vapor lamps. More particularly, it relates to an improved method for forming the laminations of such magnetic cores.

Cores of the multiple coil type transformers, having one or more separate coils, are generally constructed in laminated form, with each lamination layer being formed of two or more pieces to constitute a magnetic circuit. One common form of lamination layer comprises a pair of E and I-shaped laminations. The E-shaped lamination contains two outer legs and an inner leg joined by a cross bar at one end; and the open end of the E-lamination is closed, when assembled, by the generally I-shaped lamination.

Such laminations are generally punched from metal strips or sheets. It is desirable that for a given electrical size or rating of the transformer, the gross area of the Strip of metal from which the core lamination punchings are made be at a minimum, and that the percentage of scrap or loss of strip material from which the core laminations are punched be also at a minimum. When it is realized that cores of this type are made by the hundreds of thousands, it will be appreciated that savings in material are highly important.

In the operation of mercury vapor lamp transformers, particularly those utilizing the constant-wattage or stabilized principle of operation, portions of the core may be operated at such high fiux densities that the core exceeds the temperatures approved by the Underwriteris Laboratory. Reducing the temperature may be accomplisihed by increasing the amount of iron in the non-critical operating por/tions of the magnetic circuit in order to reduce the fluX density therein. Unfortunately, this procedure when used alone is wasteful of materials and, as implied above, transformer manufacturers are guided largely by considerations of cost.

Mercury vapor lamp transformers or ballasts are normally of relatively small size, and are of the dry-type as distinguished from oil filled transformers. Such drytype transformers are normally encased in an outer enclosure in a potting compound, such as asphalt and silica or like compound.

A core lamination construction which would allevate this core heating problem encountered in certain of the mercury vapor lamp transformers would therefore be desirable.

A general object of the present invention is to provide an improved method |of forming a magnetic core for an inductive device.

It is another object of the present invention to provide an improved method for the effective utilization of the material from which core laminations are punched so that substantially no Waste results. 4

A further object of the invention is to provide a method for forming a core of the E and I-shaped laminations wherein the laminations are so formed that they may be ice stamped out of strip material with a minimum of Waste and a maximum utilization of the strip material in the magnetic circuit.

Briefly stated, the improved method of one embodiment of my invention involves the step of forming E-laminations from a continuous strip of material Wherein the E-laminations are arranged on 4the material with two E-laminations end-to-end with each other. Each of the laminations are formed with a wide outer leg, a center leg of substantially the same width and a narrow outer leg of about one-half the Width of the wide -outer leg. In the stamping operation, the wide outer leg and center leg of the laminations are cut out from the strip to form the windows of the cooperating E-laminations. The outer edges of the strip form one edge of the narrow outer leg.

In another aspect of my invention a magnetic core is provided by a method which, in addition to the steps of forming the E-shaped laminations as described above, includes an improved step of assembling the E-shaped laminations with the I-shaped laminations. These steps involve stacking groups of E-shaped laminations having their narrow and wide outer legs respectively aligned. The groups of laminations are assembled to form a magnetic core with the wide outer legs of one group reversed with respect to the wide outer legs of the adjacent group and with the wide `outer legs of one group extending out- Wardly beyond the narrow outer legs of the adjacent group to form cooling flutes. The stacked groups of E- shaped laminations are then assembled with groups of I- shaped laminations to form a rigid core assembly.

Such a transformer core is stacked in the desired manner, for example, with the primary and secondary winding spaced apart and having a magnetic shunt therebetween. The laminations may be arranged so that one or more bridged air gaps are provided. Furthermore, the member formed of I-shaped laminations may consist of a straight bar or may contain an offset center portion thereby resultng in an air gap; a combination of these types of I-members may be used in a single core, if desired, to produce desired reluctance paths.

The invention, together with additional objects and ad- Vantages thereof, will be best understood from the following description of specific embodiments, when read in connection With the accompanying drawings, in which:

FIGURE 1 is a pictorial view of an assembled laminated core according to one embodirnent of the invention;

FIGURE 2 is an exploded pictorial view of a core illustrating the inter-relation of the reversed core laminations;

FIGURE 3 is an elevation view, in section, illustrating the core assembled with the coils;

FIGURE 4 is a plan view, partly in section, illustrating the potted transformer;

FIGURE 5 is a plan view of the layout of the E-larninations as they are punched from the strip of the material; and

FIGURE 6 is a plan view of the layout of one embodiment of I-laminations as they are punched from the strip of material.

Referring to FIGURE l, there is illustratedv a core 1 composed of laminations la, lb, le, etc., according to the instant invention arranged in a plurality of adjacent groups 2 forming a series of cooling flutes 2,' extending outwardly therefrom. Each cooling flute is composed of a group 2 of laminationsA in aligned position; adjacent groups of laminations are reversed with respect to each other, as hereinafter more fully described, thereby forming the respective cooling flutes 2' on each side of the core. The stack of laminations may be held together by through bolts 3 and angle members 4. A series of bridged` air gaps 5 may, if desired, extend through a leg of the core.

Referring to FIGURE 2, each layer of laminations includes an E-member 6 or 7 and an I-member 8 or 9. Each E-lamination 6, 7 comprises a wide outer leg 10, 11 and a center leg 12, 13 of substantially the same width and of the same width as the space or window between the legs. As more clearly indicated, by distance d, FIGURE 5, each E-lamination 6, 7 also contains a narrow outer leg 14, 15 of substantially less width than the other or wide outer leg 10, 11, or the center leg 12, 13 as indicated by distance X, FIGURE 5. X may be, for example, equal to d/ 2; however, X is not limited to this dimension. Each E-lamination also contains a cross member 16, 17 integral with the outer legs and center leg. As previously stated, each E-lamination 6, 7 may contain one or more bridged air gaps 5; in such a case it will be noted that in alternate reversed groups of laminations, the bridged air gaps 5 are likewise in a reversed position so that in lamination 6 the bridged air gaps 5 are at the end of the center leg 12 remote from the cross member 16, and in laminations 7 the bridged air gaps 5 are at the end of the center leg 13 nearest the cross member 17. In this manner, as most clearly seen in FIGURE 2, the bridged air gap 5 of the respective groups of laminations will be aligned (after assembly into a core) so that they form a single passageway therethrough. The narrow leg of each E-lamination has a recess 18 (FIGURE 5) on its inner face at the end remote from the cross member; the recess 18 comprises an inner face 19 and a shoulder 20 in line with the end edges of the wide outer leg and the center leg, the purpose of Which will hereinafter be explained.

Each layer of laminations also has an I-member associated with the E-lamination. The I-lamination may consist of a straight bar 9, FIGURE 2; or may consist of a bar having a recessed or offset center portion 21. The I-member rests on the shoulder 20 of its associated E- lamination, butting against the inner face 19 of the recess 18. The other end of the I-lamination overlaps and butts against the end edge of the wide outer leg. Where an I- lamination 8 having an offset portion 21 is used, an air gap 22 is formed with the center leg; if a straight bar is used for the I-member 9, FIGURE 2, then no air gap is formed. As assembled, core 1 may consist of I-members each having an offset center portion 21, or may consist of I-members each comprising a single bar 9, FIGURE 2, or alternate reversed groups of laminations may comprise alternate forms of I-members so that (as illustrated in FIGURE 2) each assembled core contains both forms of I-members. In this manner, the desired reluctance may be interposed in the magnetic circuit of the core.

Referring now to FIGURE 3 which illustrates the core and winding assembly of a transformer according to the instant invention, the transformer includes a core assembly 1 having a primary winding 23 and a secondary winding 24 around the center legs of the E-laminations and extending through the windows or open spaces of the E- laminations. As illustrated, to increase the reactance of the transformer and thereby to limit the current flow to the lamp by the transformer, the primary winding 23 is spaced apart from the secondary winding 24 and a magnetic shunt 25 is placed at least partially between the respective coils, as is well known in the art. The magnetic shunt 25 comprises a rectangular stack or bar of magnetic laminations and fills a portion of the window space of the laminations between the coils.

Transformers of the size employed in mercury vapor lamps are commonly, but not necessarily, potted in an enclosing case as illustrated in FIGURE 4. Such a potted transformer comprises the core assembly 1, the primary winding 23 (not shown in FIGURE 4) and the secondary winding 24 within an enclosure 26 and held in place with electrical insulating or potting compound 27 such as the heretofore mentioned asphalt and silica. Leads 28, 29 and 30, 31 of the primary winding 23 and the secondary winding 24 respectively are passed through an opening or aperture 32 in the enclosure 26.

Transformers, according to the instant invention, were built and compared in operation to similar transformers without the cooling flutes. It was found that the cooling flutes in air provided a 6% reduction in the maximum core temperature, a 12% reduction in the primary coil temperature rise, a 12% reduction in the secondary coil temperature rise, and a 14% reduction in copper (through permissible Smaller gauge wire) and a 34% reduction in the gross weight of magnetic material (as hereinafter explained). The coolng flutes in potting compound provided a 26% reduction in the primary coil temperature rise, a 27% reduction in the secondary coil temperature rise, a 14% reduction in copper needed and a 34% reduction in the gross Weight of magnetic material needed (as hereinafter more fully described). The core temperature of the potted transformer was not measured. From this analysis, it is apparent that the cooling flutes according to the instant invention show a significant improvement in the cooling of the transformers in both air and potting compounds.

For maximum utilizaion of the sheet material from which the laminations are made, a pair of E-laminations for each of the adjacent groups is formed end-for-end with respect to each other. As shown in FIGURE 5, each pair of laminations is positioned on the sheet material so that the Wide outer leg and the center leg of each E-lamination substantially fills the window of the cooperating E-lamination, and so that the formation of the center leg and wide outer leg of one E-lamnaton forms the window of the cooperating lamination; that is, the windows in the cooperating lamination are formed by the removal of these legs. The narrow leg of each lamination extends along the outer edge of the strip. It is readily appreciated that the width of the windows and the width of the wide and center legs of the E-laminations must be equal. This arrangement results in substantially no Waste (except, of course, for the material removed from the bridged air gap and bolt holes) resulting in a reduction of 34% in the gross weight of the Strip material needed for the punchings over the corresponding prior core without the flutes.

FIGURE 6 illustrates the formation of one form of I- member from a strip of sheet material. It is noted that an elevation 33 occurs on the outer edge of the I-lamination opposite the offset portion so that the I-lamination is of substantially constant cross-section. As is appreciated, the Width of the I-lamination is equal to the width of the cross-member of the E-lamination; in this way, the narrow outer leg of the E-lamination, which is punched out in length equal to the length of the Wide outer leg plus the width of the cross-member, is also equal in length to the length of the wide outer leg plus the width of the I-lamination. The formation of the elevation and offset in one I-member forms the offset and elevation of the respective adjacent punchings.

To form a transformer and core assembly, prewound primary winding 23 and secondary winding 24 are held in proper relationship with desired magnetic shunts 25 therebetween. The alternate groups 2 of E-laminations are stacked with their narrow and wide outer legs, respectively, aligned and may then be assembled with the coils by passing the center leg of one group of E-laminations through center openings in the windngs. Adjacent groups of E-windings are assembled from opposite sides of the coil arrangement with the Wide outer legs of one group reversed with respect to the wide outer legs of the adjacent group. Groups of I-laminations are assembled with the E-laminations. After the core has been formed, the angle members 4 are assembled and the core is bolted together with through bolts 3. The transformer may be used in air or, where desired, it may be potted in an enclosure 26; with the core and coil assembly within the enclosure, the enclosure may be filled with potting compound 27, such as hot asphalt and silica, to securely retain the assembly within the enclosure.

As will be readily appreciated, the construction according to the instant invention results in reduced flux density in the cooling fiutes. Since the wide outer legs of the E- laminations are equal to the central leg in Width, and the narrow outside leg is equal, for example, to 3/2 the Width of the center leg, the narrow and wide outer legs, when adjacent to each other, have an average Width of 3/1 the central leg width. If 1/2 the total flux of the central leg branches out into each of the outer legs, outer legs having 1%, the area of the central leg will result in a fluX density in the outer leg of 2/a the fiux density in the central leg.

The reduced fluX density in the outer legs, when the central leg is operating in the saturating region, is extremely effective in reducing core losses and core heating, and when combined with the cooling flutes is very effective in cooling the core.

In accordance With the patent statutes, I have described what at present are considered to be the preferred embodiments of my invention. However, it will be obvious to those skilled in the art that various changes and modifications may be made in the disclosed structure without departing from my invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications las fall within the true spirit and scope of my invention.

What I clairn as new and desire to secure by Letters Patent of the United States is:

1. The method of forming a magnetic core for .an inductive device of the type comprising a plurality of adjacent groups of layers of E and I-shaped laminations, each E-shaped lamination having a center leg of a predetermined width, a first outer leg of substantially said predetermined width, and a second outer leg having a narrower width than said predetermined width, and said legs defining a pair of windows :for receiving a coil, said method comprising the steps of forming from a strip of material said E-shaped laminations arranged on the strip of material With two E-shaped laminations end-toend with respect to each other With said first outer leg and center leg of one E-shaped lamination positioned on said strip substantially within the windows of the other E- shaped lamination, the formation of said first outer leg and center leg of the first E-shaped lamination forming the windows of the other E-shaped lamination, forming said I-shaped laminations, stacking groups of said E- shaped laminations having their first and second outer legs, respectively, aligned, assembling adjacent groups of E-shaped laminations to form the magnetic core with said first outer legs of one group adjacent said second outer legs of the adjacent group and with said first outer legs of one group extending outwardly beyond said second outer legs of the adjacent group thereby forming cooling fiutes on said core, and assembling groups of said I-shaped laminations with said E-shaped laminations.

2. The method of forming a magnetic core with E- laminations of the type having a wide outer leg, a center leg of substantially the same width as the wider outer edge and a narrow outer leg of about one-half the width of said wide outer leg, said outer legs and center leg joined by a cross member, said legs defining windows therebetween, said narrow outer leg being in length equal to the length of said outer legs plus the width of said cross member, said method comprising the steps of forming from a strip of material the E-laminations arranged on said material with two E-laminations end-to-end with respect to each other and said wide outer and center leg of one E- lamination forming the windows of the cooperating E- laminations, said narrow outer leg extending along one edge of said strip; the formation of said wide outer leg and center leg forming said windows in said cooperating E-lamination, forming I-larninations for assembly with the E-laminations, said I-laminations formed with an elevation and an offset portion, said elevation portion formed from the material cut out from the offset portion of an adjacent lamination, assembling a first group of E and I- laminations in stacked array, assembling a second group of E and I-laminations in stacked array with said first group with the Wide outer legs of the second group reversed with respect to the wide outer legs of the first group, and similarly stacking additional groups of E and I-laminations, alternate groups of E and I-laminations being stacked with the wide outer legs reversed with respect to the wide outer legs of an adjacent group to define cooling fiutes on two sides of the magnetic core and with the elevation portions forming cooling fiutes on two other sides of the magnetic core.

3. The method of forming a magnetic core of E-shaped and I-shaped laminations defining la pair of windows for receiving at least one coil, said method comprising the Steps of forming from a strip of material E-laminations having a wide outer leg, a center leg of substantially the same width as the wide outer edge and a narrow outer leg, said E-laminations positioned on said strip in end-to-end relation with respect to each other so that said wide outer legs and center legs of each lamination substantially fill the window of the cooperating lamination, the formation of said wide outer leg and center leg on one lamination providing the windows of the cooperating lamination, forming the I-laminations with an elevation and an offset portion, said elevation portion formed from the material cut out from the offset portion of an adjacent lamination, assembling a first group of E and I-laminations in a stacked array, assembling a second group of E andl I- laminations in a stacked array on said first group with the wide outer legs of the second group being reversed with respect to the wide outer legs of the first group, and similarly assembling additional groups of E and I-laminations with each alternate group being reversed with respect to an adjacent group to form a magnetic core of the desired height, with the wide outer legs forming cooling fiutes along two sides of the magnetic core and with the elevation portions forming cooling fiutes along the other two sides of the magnetic core.

References Cited by the Examiner UNITED STATES PATENTS 466,937 1/92 Pfannkuche 336--60 509,770 11/93 Scott 336--214 567,237 9/96 Hobart 336-60 2,137,433 11/38 Wirz 336-21 X 2,489,977 11/49 Porter 129-155.61 2,734,176 2/56 Gould et al. 336-212 X 2,811,203 10/57 Garbarino 29-155.61 X

WHITMORE A. WILTZ, Primary Examner.

JOHN F. CAMPBELL, Examiner. 

1. THE METHOD OF FORMING A MAGNETIC CORE FOR AN INDUCTIVE DEVICE OF THE TYPE COMPRISING A PLURALITY OF ADJACENT GROUPS OF LAYERS OF E AND I-SHAPED LAMINATIONS, EACH E-SHAPED LAMINATIN HAVING A CENTER LEG OF A PREDETERMINED WIDTH, A FIRST OUTER LEG OF SUBSTANTIALLY SAID PREDETERMINED WIDTH, AND A SECOND OUTER LEG HAVING A NARROWER WIDTH THAN SAID PREDETERMINED WIDTH, AND SAID LEGS DEFINING A PAIR OF WINDOWS FOR RECEIVING A COIL, SAID METHOD COMPRISING THE STEPS OF FORMING FROM A STRIP OF MATERIAL SAID E-SHAPED LAMINATIONS ARRANGED ON THE STRIP OF MATERIAL WITH TWO E-SHAPED LAMINATIONS END-TOEND WITH RESPECT TO EACH OTHER WITH SAID FIRST OUTER LEG AND CENTER LEG OF ONE E-SHAPED LAMINATION POSITIONED ON SAID STRIP SUBSTANTIALLY WITHIN THE WINDOWS OF THE OTHER ESHAPED LAMINATION, THE FORMATIN OF SAID FIRST OUTER LEG AND CENTER LEG OF THE FIRST E-SHAPED LAMINATION FORMING THE WINDOWS OF THE OTHER E-SHAPED LAMINATION, FORMING SAID I-SHPAED LAMINATIONS, STACKING GROPS OF SAID ESHAPED LAMINATIONS HAVING THEIR FIRST AND SECOND OUTER LEGS, RESPECTIVELY, ALIGNED, ASSEMBLING ADJACENT GROUPS OF E-SHAPED LAMINATIONS TO FORM THE MAGNETIC CORE WITH SAID FIRST OUTER LEGS OF ONE GROUP ADJACENT SAID SECOND OUTER LEGS OF THE ADJACENT GROUP AND WITH SAID FIRST OUTER LEGS OF ONE GROUP EXTENDING OUTWARDLY BEYOND SAID SECOND OUTER LEGS OF THE ADJACENT GROUP THEREBY FORMING COOLING FLUTES ON SAID CORE, AND ASSEMBLING GROPS OF SAID I-SHAPED LAMINATIONS WITH SAID E-SHAPED LAMINATIONS. 